conlang: Introduce as casting

Arbitrary primitive type conversion

Currently implemented as jankily as possible in the interpreter, but it works alright™️

compiler/cl-parser/src/error.rs

@@ -99,6 +99,7 @@ pub enum Parsing {
     BinaryKind,
     Unary,
     UnaryKind,
+    Cast,
     Index,
     Structor,
     Fielder,
@@ -204,6 +205,7 @@ impl Display for Parsing {
             Parsing::BinaryKind => "a binary operator",
             Parsing::Unary => "a unary expression",
             Parsing::UnaryKind => "a unary operator",
+            Parsing::Cast => "an `as`-casting expression",
             Parsing::Index => "an indexing expression",
             Parsing::Structor => "a struct constructor expression",
             Parsing::Fielder => "a struct field expression",